The present invention relates to miniature gas turbine engines and in particular to shaft couplings for rotor assemblies comprising turbine wheels and compressors. Miniature gas turbine engines operate in much the same ways a conventional gas turbine engines in that fuel is combusted in a source of compressed air to generate high energy gases for producing thrust and rotating turbines. As with conventional gas turbine engines, the turbines are used to turn a compressor to compress air for the combustion process, turn fan blades or eject gases at high velocity to produce thrust, or turn a generator for operating electrical systems. However, miniature gas turbine engines are much smaller such that they are readily applicable as propulsion systems for small vehicles. For example, miniature gas turbine engines producing approximately 30 lbf (˜133.5 N) of thrust or more are often used as propulsion systems for aircraft, such as reconnaissance drones, or projectile weapons, such as cruise missiles, and air-launched and ground-launched weapon systems. Miniature gas turbine engines extend the range of these aircraft and weapons beyond what is traditionally available from conventional rocket engines. Due to the usually exigent and expendable circumstances in which these aircraft and weapons systems are used, it is desirable to have miniature gas turbine propulsion systems that are at the same time reliable and cost effective.
The rotor assembly, i.e. the shaft coupling between the turbine and the compressor, in the miniature gas turbine engine affects both reliability and manufacturing cost. The compressor and turbine together comprise the main rotating unit within the miniature gas turbine engine that rotates at very high speeds while being subject to wide ranging temperatures. Due to thermal expansion and the high rotational speed, any misalignment or uncoupling of these components has the potential to produce instability during operation of the engine. The compressor typically comprises a wheel having a bore that is fitted onto a shaft integrated with the turbine. The compressor and shaft are conventionally coupled together through a simple radial interference fit. The interference fit requires that shafts and compressor bores be precision machined such that the components can be matched to form an interference fit having the desired torque transmitting capabilities. Thus, radial interference fits for shaft couplings are not conducive to easy, cost-effective manufacturing on a large scale, which makes miniature gas turbines less attractive for expendable applications. Furthermore, during operation of the miniature gas turbine, thermal growth and centrifugal expansion of the compressor wheel has the potential to cause separation from the turbine shaft, thus causing balance instabilities and making the shaft coupling less reliable than desired. In worse case scenarios, the compressor wheel may burst at the interference fit due to heating of the shaft and turbine during operation of the engine. There is, therefore, a need for a more reliable and inexpensive coupling mechanism for use in miniature gas turbine engines.